1. Field of Invention
This invention relates generally to electronic compasses and more particularly to a method and system for improving electronic compass calibration.
2. Description of Related Art
Electronic compasses are well known in the art for determining a geographical direction by detecting the Earth's magnetic field. Electronic compasses use magnetic field sensors to detect the Earth's magnetic field. The Earth's magnetic field may be described by the direction and intensity of the magnetic field. The direction and intensity of the magnetic field can be identified by vector components. At the equator, the Earth's magnetic field is entirely a horizontal vector component with no vertical (up and down) vector component. At positions north or south of the equator, the Earth's magnetic field consists of both horizontal and vertical vector components.
Electronic compasses are used in a variety of applications such as automobiles, aircraft, marine vehicles, and personal handheld devices. The level of compass accuracy in calculating a geographical direction varies per application. Lower levels of compass accuracy can be achieved using a compass with two sensitive axes. Such a compass is required to be kept level and any tilt will result in loss of compass accuracy. Higher levels of compass accuracy can be achieved using a compass with three sensitive axes. These compass implementations use some type of gravity sensing device to maintain compass accuracy with tilt. The three sensitive axes in a three-axis compass are commonly referred to as the x-axis, y-axis, and z-axis. Typically, the x-axis and y-axis sensitive axes are arranged to detect the horizontal vector component of the Earth's magnetic field and the z-axis sensitive axis is arranged to measure the vertical vector component of the Earth's magnetic field. The three sensitive axes of a three-axis compass are typically arranged to be mutually orthogonal (at right angles). Each sensitive axis of an electronic compass is contained in a magnetic field sensor. A magnetic field sensor may include more than one sensitive axis. A variety of magnetic field sensors are available for electronic compassing. Examples of magnetic field sensors in electronic compasses include flux-gate sensors, Hall-effect sensors, and magnetoresistive sensors.
The Earth's magnetic field is susceptible to distortion from other magnetic sources. For example, hard and soft iron effects from a variety of magnetic sources may distort the Earth's magnetic field. The distortion of the Earth's magnetic field can cause an electronic compass to determine a geographical direction that does not reflect the true geographical direction. Calibration procedures for electronic compasses have been developed in order to compensate for the hard and soft iron effects that distort the Earth's magnetic field. The calibration procedures depend on a variety of factors, such as the type of compass application (car, boat, hand-held, etc.) and/or the number of magnetic field sensor sensitive axes in the compass.
The quality of a compass calibration procedure depends in part on the process used to obtain calibration data. A process for obtaining calibration data typically involves rotating the compass in a specific manner. By rotating the compass, a sensitive axis in the compass may experience a variation of the magnetic field acting on the sensitive axis. The variation of the magnetic field refers to changes in the intensity and directionality of a magnetic field arising from the sensitive axis moving in relation to the magnetic field acting on the sensitive axis.
The currently known calibration procedures that provide a variation of the magnetic field in the sensitive axes have limitations. For example, a first compass calibration procedure for acquiring calibration data includes pointing a compass in three known directions. A limitation of the first compass calibration procedure is that the Earth's magnetic field is sampled in only three directions. Another example is a second compass calibration procedure for acquiring calibration data that includes calibrating a compass by moving the compass in a complete circular path. The second compass calibration procedure may be acceptable for a compass with two sensitive axes because a variation of the magnetic field in the two sensitive axes can be achieved. However, the second calibration procedure does not guarantee that all three sensitive axes of a three-axis compass will experience variation while moving the compass in the complete circular path. Yet another example is a third compass calibration procedure for acquiring calibration data that includes rotating the compass at a steady speed through 360 degrees with as many pitch and roll orientations as possible. The pitch and roll orientations require moving the compass so that the sensitive axes of the compass and a gravity vector form an angle that varies while the compass is rotating through the 360 degrees. Providing the pitch and roll orientations to achieve a varying angle increases the complexity of acquiring calibration data. One more example is a fourth compass calibration procedure for acquiring calibration data that includes rotating a three-axis compass with three mutually orthogonal sensitive axes in two circular paths. The first circular path of the fourth compass calibration procedure is a circular path in a horizontal plane when a first pair of sensitive axes is in the horizontal plane. The second circular path of the fourth compass calibration procedure is a circular path in the horizontal plane when a second pair of sensitive axes is in the horizontal plane. Changing from the first pair to second pair of sensitive axes in the horizontal plane typically involves rotating the compass 90 degrees. The two-circular-path procedure requires two rotational steps to provide a variation in all three sensitive axes. Two steps are required because the sensitive axis orthogonal to the horizontal plane in each step does not experience a variation in the magnetic field when rotating the compass in the circular paths. Instead the sensitive axis orthogonal to the horizontal plane rotates about a single point.
It would be helpful to have a method and system for acquiring calibration data for a three-axis electronic compass where each of the three sensitive axes experiences a variation in the magnetic field during the acquisition process without having to move the compass in two circular paths and/or without having to move the compass with a pitch and roll motion.